1. Field of the Invention
The present invention relates to a semiconductor laser device having an active region made of at least one aluminum-free material.
2. Description of the Related Art
Recently, the range of uses of semiconductor laser devices is extending remarkably. In particular, the range of uses of the semiconductor laser devices using a GaAs substrate and emitting laser light in the 0.7 to 1.1 xcexcm wavelength band is extending with increase in their output power. For example, such semiconductor laser devices are currently used as: excitation light sources in solid-state lasers, optical fiber amplifiers, and fiber lasers; primary light sources for second harmonic generation; light sources for laser thermal image formation on thermally-sensitive materials in the field of printing; and light sources in laser beam machining, soldering, and medical applications.
In the above applications, it is very important to increase output power of the semiconductor laser devices. As examples of narrow-stripe single-mode semiconductor laser devices having a stripe width of about 5 xcexcm or less, semiconductor laser devices which emit laser light of 0.98 and 1.02 xcexcm wavelengths, have a maximum optical output power of 500 mW or more and a practical optical output power of 150 mW or more, and are used as excitation light sources for optical fiber amplifiers, are reported. In addition, as examples of wide-stripe multiple-mode semiconductor laser devices having a stripe width of about 50 xcexcm or more, a semiconductor laser device which has a stripe width of 100 xcexcm and a maximum breakdown optical output of 11.3 W at the oscillation wavelength of 0.87 xcexcm, and a semiconductor laser device having a stripe width of 200 xcexcm and a maximum breakdown optical output of 16.5 W at the oscillation wavelength of 0.87 xcexcm are reported. (Refer to S. O""brien, H. Zhao, and R. J. Lang, Electronics Letters, vol. 34, No. 2, pp.184, 1998.)
Previously, the present applicants proposed a semiconductor laser device which achieves high output power by making a light emission region and its vicinity aluminum-free so as to prevent sudden failure due to oxidation of aluminum, and forming cladding layers with AlGaAs so as to prevent deterioration of temperature characteristics due to leakage of electrons from the active region. (Refer to T. Fukunaga, M. Wada, H. Asano, and T. Hayakawa, Japanese Journal of Applied Physics, vol. 34, No. 9B, pp.L1175, 1995.)
In the above semiconductor laser device proposed by the present applicants, an InGaP cladding layer having a thickness of 0.1 xcexcm is provided on each side of a quantum well layer (i.e., a total thickness on both sides is 0.2 xcexcm), and a laser light confinement factor xcex93 is relatively large. Therefore, when a semiconductor laser device having a stripe width of 50 xcexcm is aged at 50xc2x0 C. with a power of 500 mW in an automatic power control (APC) mode, the applicants obtained a median deterioration rate of 5xc3x9710xe2x88x925 hxe2x88x921, which is represented by the rate of increase in the driving current in the APC mode. In addition, when a semiconductor laser device having a stripe width of 200 xcexcm is aged with power of 200 mW in an automatic power control (APC) mode at 25xc2x0 C., the applicants also obtained a median deterioration rate of 5xc3x9710xe2x88x925 hxe2x88x921, which is represented by the rate of increase in the driving current in the APC mode. That is, the deterioration rate is relatively high. Since the high-output-power semiconductor laser devices having wide stripe structure stop oscillation when the driving current increases by about 5%, for example, the median value of the lifetime of the above semiconductor laser device having a stripe width of 200 xcexcm is estimated at about 1,000 hours, which is practically insufficient.
An object of the present invention is to provide a high-output-power semiconductor laser device, containing an Al-free active layer, which has a longer lifetime and higher long-term reliability than the conventional semiconductor laser devices containing an Al-free active layer.
According to the present invention, there is provided a semiconductor laser device including an active region which is made of an aluminum-free material and a plurality of cladding layers made of at least one AlGaAs or AlGaInP material. The active region includes a quantum well layer and at least one optical waveguide layer; a portion of the at least one optical waveguide layer located on one side of the quantum well layer has a thickness of 0.25 xcexcm or more; and the at least one optical waveguide layer, other than a portion of the at least one optical waveguide layer being located near the quantum well layer and having a thickness of at least 10 nm, is doped with impurity of 1017 cmxe2x88x923 or more.
Preferably, the semiconductor laser device according to the present invention may also have one or any possible combination of the following additional features (i) and (ii).
(i) Predetermined areas of at least one of said plurality of cladding layers, above a boundary between said at least one of said plurality of cladding layers and one of said at least one optical waveguide layer located below the at least one of said plurality of cladding layers, may be selectively removed so as to form a ridge structure.
(ii) The aluminum-free material may have a composition of InxGa1xe2x88x92xAsyP1xe2x88x92y (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61).
Since the thickness of the optical waveguide layer on at least one side is made 0.25 xcexcm or more according to the present invention, the optical density (xcex93/d; d is the thickness of the quantum well layer) in the quantum well portion is reduced. Since the deterioration rate (i.e., the rate of increase in the driving current in the APC mode) generally increases in proportion to the fourth power or more of the optical density, both the internal loss and the deterioration rate during APC aging can be reduced in the semiconductor laser device according to the present invention.
In addition, since the at least one optical waveguide layer, other than a portion of the optical waveguide layer being located near the quantum well layer and having a thickness of at least 10 nm, is doped with impurities (donors or acceptors) of 1017 cmxe2x88x923 or more, the resistance of the optical waveguide layer is reduced.
In addition to the above provision, the active region is made of an aluminum-free material. Therefore, the lifetime of the semiconductor laser device according to the present invention is remarkably increased, and the long-term reliability is also increased.
In particular, the semiconductor laser device according to the present invention realizes high quality index-guided semiconductor laser device, and there is little quality deterioration due to aging. Therefore, the semiconductor laser device according to the present invention can increase reliability of a system which handles images when the semiconductor laser device according to the present invention is used as a light source in the system, since variations in noise, intensity, and beam shapes of the light source are serious matters in such a system which handles images.
Typical examples of applications of the semiconductor laser devices as light sources are printers and image scanners in which visible or ultraviolet light sources are each constituted by a semiconductor laser device and a second harmonic generator. For example, in thermal printing systems using thermal sensitive material and being used in the fields of medicine and printing, semiconductor laser devices per se or semiconductor laser devices coupled with optical fibers are used as light sources for direct exposure in thermal printing. In such thermal printing systems, a few to a hundred high-output-power semiconductor laser devices are used in each system. Therefore, the long-term reliability of the semiconductor laser device according to the present invention can greatly increase reliability of the thermal printing systems.
Conventionally, waveguide loss of laser light in a semiconductor laser device was considered to increase due to free-carrier absorption by the remaining carriers. Therefore, conventionally, the carrier densities in thickened optical waveguide layers were minimized, and no special doping was performed, for example, as disclosed in U.S. Pat. No. 5,818,860. That is, the doping of thickened optical waveguide layers with impurities of 1017 cmxe2x88x923 or more has not been proposed before the present invention.
Further, the semiconductor laser device according to the present invention has an index-guided structure, where the cladding layers are made of AlGaAs or AlGaInP, and the optical waveguide layer is made of an aluminum-free material such as InGaAsP. Therefore, it is possible to etch one of the cladding layers to the boundary between the cladding layer and the optical waveguide layer due to the difference in the etching rate between the cladding layer and the optical waveguide layer.
In the above construction, a portion of the optical waveguide layer is in contact with an insulation film or a current confinement layer or a regrowth boundary surface. Therefore, there is a possibility of deterioration of the semiconductor laser device due to nonradiative recombination at such a boundary surface. Nevertheless, when the optical waveguide layer is doped according to the present invention, the density of injected minority carriers decreases, and the lifetime of the semiconductor laser device is increased.
Furthermore, according to the present invention, the optical waveguide layer is further thickened, compared with the conventional semiconductor laser device. Accordingly, the penetrated amount of evanescent light into the cladding layer is reduced. Therefore, absorption in the cap layer can be reduced even when the thickness of the upper cladding layer is decreased. That is, the thickness of the upper cladding layer can be 1 xcexcm or less, while the conventional upper cladding layers must have thicknesses of 1.5 xcexcm or more. When the upper cladding layer is made thin, unevenness caused by etching for producing the index-guided structure can be reduced. Therefore, a lithography process becomes easier, and accuracy of the lithography is increased. Thus, since unevenness of the surface of each complete semiconductor laser device can also be reduced, soldering material can be uniformly laid at the time of chip bonding. Therefore, the heat dissipation characteristic of the semiconductor laser device is also improved.